China's medium frequency electric furnace industry is developing rapidly, and technically, the performance of domestic equipment has approached the world's top level. A typical application is the one-rectifier-two-inverter variable frequency furnace: one rectifier drives two inverters to power two electric furnaces of the same specification for quenching equipment. This allows the power of one variable frequency power supply to be flexibly distributed to two furnace bodies. Specifically, the major power of the power supply is allocated to the melting furnace, and the remaining small power is allocated to the holding furnace, so as to realize the operation mode of "one for melting, one for heat preservation", and enable the electric furnace system to produce molten iron continuously.

1. Research on Load Matching

Whether the load impedance matching is proper directly affects the rated power of the power supply. The design and calculation of the induction furnace coil (load) are very complicated, and it is no easy task to design a satisfactory load coil. At present, many designers in China often ignore secondary parameters or design with experience-modified formulas, which leads to large errors.

2. High Power Factor and Low Harmonic

The new generation of power supply must feature high power factor and low harmonic. The currently developed medium frequency furnace technologies include multi-level rectification technology, fully-controlled power tube combined with matrix control or PWM control, series circuit, chopping technology, etc. It has also promoted the R&D and production of harmonic elimination devices for power harmonic filtering and power factor compensation.

3. Energy Conservation and Emission Reduction

The electrical efficiency of a well-designed medium frequency induction furnace can usually reach over 78%. Therefore, it is unlikely to achieve a substantial improvement in the melting efficiency of induction electric furnaces under the existing technical concept. To realize remarkable energy conservation and emission reduction, it is necessary to make innovations on the inductor and charging system, break through the existing framework, establish a brand-new design model, and lift the system electrical efficiency to over 90%.

4. Temperature Control Technology

The medium frequency melting furnace adopts CPU computing program and complex software compiled by high-level programming language. Its basic function is to continuously monitor the output power of the medium frequency power supply, count the energy input into the electric furnace, and continuously monitor the ambient temperature, cooling water temperature and flow rate, so as to calculate the energy consumed by the electric furnace.

5. Constant Power Control Technology

Series resonant power supply can realize impedance adjustment by adjusting the working frequency of the medium frequency power supply. The change of working frequency can directly change the impedance of the electric furnace. When the impedance of the electric furnace is too low, the working frequency can be appropriately increased; when the impedance is too high, the working frequency should be decreased. Through this adjustment, the impedance of the electric furnace remains basically consistent throughout the melting process, so as to ensure that the medium frequency power supply always works at the rated power.